Minimalism is a popular design term and aesthetic, but it seems to mean something rather different to a lot of different people. Generally when people talk about minimalism, they are referring to a design of theirs that has a varying amount of minimalism. This is fine, but let's flesh out exactly what minimalism is in its pure form, because it's a useful design technique even if a "pure minimalism" is not necessarily called for in what you're working on.
Distilling Minimalism To Its Essence
Here are the precise requirement for pure minimalism, so that we have a solid base to talk from:
1. Everything you need must exist
2. Nothing you don't need should exist
3. There should be no redundancy
I'll tackle these one at a time, though they may largely seem self-explanatory.
The first principle is probably the most important, because it's the one most frequently violated by those pursuing minimalism. Minimal design means finding a way to encapsulate all needed functionality in the smallest "footprint" (for the space in question, whether floor space or screen space or whatever else) possible. If you make a conventional chair but then remove one leg, you aren't exercising minimal design--you're making a bad, unbalanced chair. Unless, of course, you come up with a new design to get the same functional power out of three legs.
The second principle is the one that people think about most with minimal design and requires the least explanation. Anything that doesn't add necessary functionality must go!
Finally, you can technically adhere to both of those two principles and still break minimalism by, to put it simply, adding the same necessary functionality twice. One could argue that this is already implied in (2) because a second version of the same thing is not needed, but I like to make this point explicitly, because it's one of the easiest pitfalls for designers.
Minimalism In Your World Of Warcraft User Interface
Let's look at these principles applied to a customized UI.
The least-cluttered interface is also the quickest non-default UI to create. Hit Alt-Z and marvel at your clean, unfettered display of the game. "Perfect UI, right?" I ask rhetorically. "Of course not, don't insult my intelligence by rhetorically assigning me, the reader, such an absurd position," you respond with some asperity. "My apologies, that was a bit condescending, I suppose, and lazy, generic writing to boot," I allow cautiously.
Clearly, there are some rather vital pieces of information that a real UI must incorporate. Your resources, like health, mana, energy, rage, runic power, etc, are pretty important to monitor. Decisions are generally based on the state of these resources and none of our brains are set up to process in-game events to precisely keep track of those resources in our heads. The resources of the enemies that we're fighting are also vitally important, because many of our decisions are made based on the state of those enemies. While this may seem like stating the obvious, and is obvious in these examples, this is the type of analysis one needs to do in determining what is necessary and what is not. What decisions are made based on the piece of information? How important are those decisions? How quickly do those decisions need to be made to be valuable? Are they in-combat decisions, or can they be made out-of-combat?
You can start from a cluttered interface and pare down, or start from a blank(-ish) slate and work your way up. Either way, you need a process for determining what is essential.
Once you've done that, principle two is pretty easy to execute: everything else needs to go.
Principle three is the final pass you need to make. Is any piece of information duplicated? This can be as simple as having both a player frame displaying your health and a raid frames add-on that includes you, therefore showing your health in two places (more on this later, though). Do you have your opponent's debuffs displayed in addition to Power Auras that give you visual alerts for the key debuffs you need to watch? In order to be perfectly minimal, you want to pare away the duplications.
Is Pure Minimalism Good?
So congratulations on your perfectly minimal design. It has every piece of information you require to do your job correctly, it has absolutely nothing that isn't essential and no information is duplicated. It was a grind but success is sweet.
Or is it? (Masterful twist.) Are you happy with this super-minimal UI? Everything's there, but is it easy to quickly process and translate into action? Does it look nice?
In the end, minimalism is a concept and aesthetic...it is not always, or even often, a desirable destination in its pure form. Maybe having a larger player frame in addition to your raid frames is a good thing. You may be able to customize your raid frames to show mana bars, but maybe the resulting mana bar in your raid frame box is too small to easily see. Maybe it's simply more important to see your own health than everyone else's and making your health exactly as visible as everyone else's costs you precious split-seconds in making a decision. Maybe you just think it looks nicer to have a proper player frame to create visual balance next to your target's frame. These are all examples of perfectly valid reasons to break away from a purely minimalistic design.
It's good to keep minimalism in mind. The cleaner and simpler your interface is, the less it will get in your way...to a point.Make it too minimal (and this still means in accordance with the principles laid out above) and it can start to get in your way again, by making it harder for you to process that necessary information. I think it's valuable to understand what, precisely, minimalism is...but a UI should balance minimalism, attractiveness and ease of use. There are often trade-offs to be made between those three things and what the right balance of those things are for you will be different than for someone else.
Final pro tip: Reduce all fonts in your UI to an unreadable 4 pt and bask in the minimal glory!